Hard drive failure got me a fusion drive
1 - 2 horas
The original 1TB hard drive from my iMac started to make weird clicking noises, I was pretty sure it was going to fail one day or another. So I decided to not just replace it with another similar, but to put a 2TB hard drive along with a 256GB SSD, replacing the disc drive which was useless for a long time.
I bought the correct tools and opening the iMac was easier than I thought it was. The suction cups removes perfectly the front glass, and the toolkit has just the right bits for every screw. The tricky part was to lift the display and disconnect all four cables connected to the logic board, but with patience it is quite easy to do. After it was entirely opened, the HD replacement was as easy as unplugging one and plugging the other, no secret there.
During the reassembly, after I closed everything, I turned on the iMac and the display was blinking and with some noises. I opened it again and discovered that the main display cable was not perfectly attached, so I reconnected it and put it all together again and it worked perfectly. So my advice is to make sure that all cables are perfectly connected before reassembling your iMac.